1. Field of the Invention
The present invention relates to a thin copper foil for a printed wiring board and more particularly to the thin copper foil with a peelable copper foil supporter, which has stabilized a strength between the supporter and the thin copper foil having a thickness less than 12 .mu.m.
2. Description of the Prior Art
Recently, circuits in a printed wiring board assembled in electronic machines are going to high density and electric conductors of the circuits tend to be extremely fine according to rapid progress of miniaturization of the electronic machines.
To fulfill these requirements, it is common to use a thin copper foil having a thickness of less than 12 .mu.m wherein side etching seldom occurs. Since these thin copper foils are weak, it is difficult to use them as they are easily creased or torn in handling. Therefore, as a copper foil for an outer layer of a multilayer printed wiring board, a thin copper foil with a supporter is used, including an aluminium foil or a copper foil. A thin copper foil layer is electrically deposited on one side of the supporter, this thin layer having a thickness of less than 12 .mu.m.
As an example, it has been proposed to use a supporter which is prepared by depositing a chrome metal on a surface of a side of an electrolytic copper foil by means of chrome plating or the like. A thin copper foil layer is then deposited on the chrome metal so as to make a thin copper foil with a peelable deposited copper foil surpporter (See Japanese patent publication Sho. 53-18329).
As another example, it has been proposed to use a four layer metal wiring foil. The supporter comprises copper, and a chromate film which is electrolytically formed on the shiny side of the copper foil supporter layer as a parting layer. The wiring foil also includes a metal "release" layer of copper-nickel alloys formed on the chromate release layer and a further thin copper non-self supporting layer in the metal "release" layer. The 3 layers composed of the copper foil supporter, the chromate film and the metal "release" layer are released except for the thin copper foil layer of the outer layer after laminating on a substrate. (See Japanese patent Sho. 57-20347.)
To make a multilayer board by using the thin copper foil with the supporter, it was carried out through such steps as shown in FIG. 2.
First, a prepreg comprising glass and epoxy was laid on both sides of a printed wiring board for inner layer and a copper foil with a supporter is laid on the outer side of the prepreg to face the copper foil supporter outwardly and then these were laminated one another under high pressure and temperature. The finished multilayer board were drilled at desired positions thereof and then the copper foil supporter was peeled off mechanically. Thereafter, in order to form an electric conductor in each of the drilled holes, electroless plating was applied thereto after a step of pretreatment. Next, the portions of the board except the circuit portions to be necessitated were covered with a photoresist. A thin copper plating was given to the circuit portions and the electric conductor in the hole using a copper electroplating bath for through-holes and soldering was carried out thereon and thereafter the photoresist was removed. Finally, the laminated board was etched with an etching liquid of alkaline ammonia using a quick etching method so as to produce a multilayer printed wiring board.
However, there remained some troubles when producing the thin copper foil with the supporter and also when producing a multilayer board by using same.
For instance, since a peel strength between the copper foil supporter and the thin copper layer, was dispersed widely and therefore the following troubles occurred to the thin copper foil with the supporter.
Where the peel strength was too weak, there were the following drawbacks.
1 When the thin copper foil with the supporter was laminated with the prepreg under high pressure and temperature and at the time of taking out its laminated board from a press machine or at the time of cutting same in a desired size, the copper foil supporter was peeled off naturally and therefore there was the danger that the thin copper foil layer on the surface of the laminated board would be damaged during handling of the laminated board.
2 In the process of drilling the laminated board, when a drill was withdrawn from the drilled hole, the copper foil supporter was peeled off and lifted from the board. As the result, the drilling process could not be continued.
Conversely, when the peel strength of the supporter was too strong, there were the following disadvantages.
3 After drilling, when the supporter was peeled off from the board mechanically, a part of the thin copper foil layer was damaged or broken at an end thereof located around the holes.
4 Particularly, in a case of a large laminated board a large force is required to peel the supporter off from the board.
5 In the case of Japanese patent 53-18329, a part of the chromate plating layer would sometimes be left on the thin copper foil layer after peel of the supporter. Also, in the case of Japanese patent 57-20347, the copper-nickel compound layer would sometimes be on the thin copper foil layer, as shown in FIG. 4. As the result, to remove the remaining layer, polishing or etching was required. However, it was practically impossible to remove the remaining copper-nickel compound layer from such a thin copper foil, and therefore the board as printed wiring with the thin copper foil including the remaining copper-nickel compound layer could not be used.
6 In the case of Japanese patent 53-18329, the supporter peeled off from the board contained a great amount of chrome, and in the case of Japanese patent 57-20347, a great amount of nickel was included. Therefore, those supporters could not be recycled.
7 In the case of Japanese patent 53-18329, to produce the thin copper foil with the supporter, it normally used a chromate plating bath of strong acid containing hexavalent chromium ion in high concentration. Therefore, treatment of the waste plating liquid was troublesome and became expensive. Further, in the case of Japanese patent 57-20347, the waste plating liquid was electrolyzed with high current density and as the result a great amount of hazardous mist was produced.
Namely, through examination of the above mentioned disadvantages in connection with the use of conventional copper foil with a supporter, the inventors have found that there occurrs dispersion of a peel strength between the chromate plating layer and the thin copper foil in the case of Japanese patent 53-18329, or a peel strength between the copper-nickel compound layer and the thin copper foil in the case of Japanese patent 57-20347, when the supporter is peeled off from the thin copper foil. This dispersion originates in the connection between the parting layer which is formed by means of chrome plating in the case of Japanese patent 53-18329 or by means of copper-nickel plating in the case of Japanese patent 57-20347, and the thin copper foil layer which is formed by means of copper plating. Further, as the other disadvantages, after the peeling off the supporter the board tends to oxidate and soft and thin copper foil layer is left thereon, the surface of the copper becomes oxidated or damaged mechanically unless the next process is applied thereto immediately.